I was about to respond with something like "
Of course it's this way...", then I realized it is very ambiguous.
Most classic'scape cards use "One at a time, roll the 20-sided die for …" when die rolls are separated. This seems to imply that Red Skull rolls the d20 only once per turn.
However, Dust of Death states "If you roll a 19 or higher,
that figure is destroyed." If you were selecting 3 figures all at once, then it should say "Those figures". Therefore, I'd conclude that you roll the d20 one at a time, once per adjacent figure you're trying to destroy.
That said, I'd easily be persuaded to believe the other way is correct. That power is worded poorly.